1. Field of the Invention
Embodiments disclosed herein generally relate to data storage systems, and more particularly, to thermally assisted recording devices.
2. Description of the Related Art
Higher storage bit densities in magnetic media used in disk drives have reduced the size (volume) of magnetic bits to the point where the magnetic bit dimensions are limited by the grain size of the magnetic material. Although grain size can be reduced further, the data stored within the magnetic bits may not be thermally stable. That is, random thermal fluctuations at ambient temperatures may be sufficient to erase data. This state is described as the superparamagnetic limit, which determines the maximum theoretical storage density for a given magnetic media. This limit may be raised by increasing the coercivity of the magnetic media or by lowering the temperature. Lowering the temperature may not always be practical when designing hard disk drives for commercial and consumer use. Raising the coercivity, on the other hand, requires write heads that incorporate higher magnetic moment materials, or techniques such as perpendicular recording (or both).
One additional solution has been proposed, which uses heat to lower the effective coercivity of a localized region on the magnetic media surface and writes data within this heated region. The data state becomes “fixed” once the media cools to ambient temperatures. This technique is broadly referred to as “thermally assisted (magnetic) recording” (TAR or TAMR), “energy assisted magnetic recording” (EAMR), or “heat-assisted magnetic recording” (HAMR) which are used interchangeably herein. HAMR can be applied to longitudinal and perpendicular recording systems as well as “bit patterned media”. Heating of the media surface has been accomplished by a number of techniques such as focused laser beams or near-field optical sources.
The optical power in the light delivery path of HAMR heads affects the heating temperature profile, and hence the recording quality during HAMR recording. During writing operations, optical power from a laser diode (LD) may fluctuate due to mode hopping, temperature drift, and LD aging. Monitoring and controlling the optical power may improve HAMR recording quality, reliability and head lifetime. During LD power fluctuation, its wavelength may also vary, causing standing wave patterns to shift. Standing wave patterns may be present in the light delivery path of the HAMR head due to reflections from the magnetic media and the near field transducer (NFT), and can cause optical coherence effects.
Therefore, an improved HAMR device that is less sensitive to or free of optical coherence effects is needed.